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hello everyone jeff cote here with boatingtechtalk.com we have a question from a fellow boater named don don asks jeff i've been told not to switch batteries while the engine is running in other words not to turn the battery switch when the engine is running is there any tr through the truth to this i'm talking about going from battery one to battery two or going from both to battery one or battery two great question by the way don battery switches actually have a little label never turn off when engine is running so first thing battery switches come in different sort of flavors right there's on off battery switches and in this instance uh don is talking about an off one two both switch or an off one to all switch so basically it's a battery switch that has three posts uh battery one battery two and the other post is common meaning you can choose to run the whole boat off battery one you can choose to run the whole boat off battery 2 or you can choose to run the hole both off both batteries the challenge is that your starter which is connected to the common post is also connected to an alternator so in turn when you have that battery switch off you should not be able to start your engine you shouldn't right that's why the battery is there and that battery switch allows you to start the engine and run house loads either off battery one or battery two the problem is that an alternator can never be disconnected from a battery while the engine is running the reason for that is that you're going to actually literally blow up the alternator it's like stopping a car with a wall it's going to work but your car is no longer going to be a car so to avoid that people have been saying don't turn the battery switch ever off now here's where it gets complicated battery switches like everything else over time will fail they do fail i see it every week there's always a service call that we go to where a battery switch has failed they don't last forever and most of us on our boats don't end up changing components until they do so what happens these switches have this concept of make before break meaning they will actually when you go from one to both they're actually gonna put both and one together effectively so that you're never you're always have you always have a connection with the battery and that's assuming that the battery switch is doing a great job so the make before break is working over time the contacts on a make before break can be not perfect if ever there is a millisecond of disconnect between the alternator which is connected to the starter solenoid which is in turn connected to the common post of the battery switch which is in turn connected to whatever battery post you've selected which is then connected to the engine battery or house battery battery one or battery two if ever that gets disconnected while the engine is running you'll lose your alternator so if you're constantly moving that battery switch from you know one both and two if your battery switch is in great shape no problem you can do it but if your battery switch has been there for 20 30 40 years and you're doing that maybe it's time to think about buying a new battery switch replacing it a little bit of preventive change because if you don't do that you might lose an alternator an alternator might be a couple hundred bucks might be more depending on the size of your engine the challenge is not changing the alternator because that takes time and you might be savvy to do it the challenge is what happens if that happens to you when you're far away you're on a long trip and now you lost your alternator losing an alternate is a big deal in a car it's a big deal and it's a big deal on a boat now you've lost the ability of recharging your batteries so now when you're underway the loads are effectively running your battery and how long can you run loads off a battery some boats might be a couple hours some other boats might be 10 hours some other boats might be 15 but eventually you need a way to recharge the batteries even when your engine is running and that's why battery manufacturers always say at the bottom of those switches never turn off while running engine but further than that if you've got an old battery switch don't wait till it dies because when it's gonna die and you turn that dial from one to both or from both to two and you lose it just for a tiny tiny period of time you're going to lose both of your alternators and that would be sad very sad so great question by the way don and thanks for asking and if any of you have comments or share your horror stories because by the way what don is asking happened on my boat testify happened in 2006 and let me tell you i learned a valuable lesson that day and here i am sharing my story with all of you so if some of you have experiences of what happened with your boat related to those battery switches failing and maybe losing an alternator educate the rest of us so that we too can learn from everyone's mistakes and make boating safer for all of us thanks for watching thank you for watching this pys video if you've got further questions please ask them below or send us an email via the contact forms on our website happy to donate my time to share information with you you can support us in keeping this channel ad free by purchasing some merchandise on our store or by making a donation on paypal and don't forget to subscribe thanks again for watching Videolytics    Tools  View advanced statistics on the video you're watching Compare this video with other videos on YouTube View and copy thi